Vehicle designers prioritize passenger safety when developing vehicle systems. By increasingly incorporating sensors into a vehicle, vehicle systems are rapidly becoming much more aware of their environment and capable of responding to detected conditions. These conditions may indicate a threat of an undesirable outcome, such as, for example, an impending collision. Responses to a sensed input indicating an impending crash or other undesired outcome may include a wide range of actions, including controlling aspects of the vehicle's chassis or suspension system, controlling propulsion systems, responsive steering and braking, tightening seatbelts, adjusting passenger seat position, closing windows, and/or improving wheel traction (typically by employing brake action or torque vectoring in the rotational direction of the wheels).
Use of systems such as anti-lock braking (ABS), traction control, and electronic stability control have been instrumental in improving vehicle safety. Despite these improvements, however, accidents do occur. In situations when vehicles are able to determine that an accident is impending or is likely to occur, it is desirable that certain avoidance measures be taken to reduce the likelihood of the occurrence of an accident or reduce its severity if it does occur. A vehicle traveling at a speed of 60 mph will travel a distance of approximately 88 feet (27 m) per second. Therefore, in order for such actions to be effective, such defensive actions need to occur with sufficient swiftness.
Over the past several years, electric power steering (EPS) has gradually been replacing hydraulically assisted power steering in modern vehicles. EPS systems offer significant advantages compared to hydraulic systems. For example, since they draw power only when needed, cars with EPS tend to have better fuel economy. For example, when the vehicle is traveling in the straight forward direction, an EPS system is largely inactive and draws little or no power. They also eliminate the need for a hydraulic fluid circuit that is cumbersome to install and maintain.
However, in EPS systems the quantity of torque assist is not determined mechanically but is purely a function of the response of the ECU to input from sensors. This arrangement may mask road feel and driving response may suffer. EPS systems also typically exhibit increased steering torque and system inertia, which tends to make the system less responsive. To correct some of these deficiencies that are inherent in EPS systems, manufacturers increasingly must rely on computer based control strategies of the EPS so as to try to mimic some operationally desirable characteristics of traditional hydraulic systems.
Sensors are used to measure parameters such as steering wheel position, vehicle speed, vehicle acceleration, vehicle yaw angle and yaw rate, and the system typically relies on computations to determine the steering wheel torque that the driver would expect, given the state of the vehicle and road characteristics. The EPS controller then commands the EPS electric motor to present the appropriate response to the driver at the steering wheel. The contents of each of U.S. Pat. Nos. 5,704,446 and 6,658,335, which describe the construction, use and control of electric power steering systems, are incorporated herein by reference in their entirety.